Silver Wain was one of the names of the Great Bear in the poem Kortirion among the Trees, written by J.R.R. Tolkien in November 1915 and later revised in many instances until the final version, now titled The Trees of Kortirion was composed, according to Christopher Tolkien, "nearly half a century after the first" (LT1, 39).
The poem was published by C. Tolkien in LT1, 33-43 in three versions, first (= KT1; LT1, 33-36) "in its pre-1937 form, when only slight changes had yet been made" (LT1, 32), second (= KT2; LT1, 36-39) "as [Tolkien] rewrote it in 1937" (LT1, 36), and third (= TK; LT1, 39-43) now renamed and composed in 1960's as explained above.
The Silver Wain replaces the Silver Bear (cf. KT1 III,120-121) in the third part of poem in its second version:
KT2 III,119-121: The seven candles of the Silver Wain, / Like lighted tapers in a darkened fane, / Now flare above the fallen year. (LT1, 39)
In the final version the lines are still slightly modified:
TK III,120-121: The funeral candles of the Silver Wain / Now flare above the fallen year. (LT1, 43)
Literature: LT1 = The Book of Lost Tales Part One (The History of Middle-earth, vol. 1)